For such a historically red state, that doesn't appear very laissez-faire.

Red states in the US don’t follow laissez-Faire economics. They follow race to the bottom and beggar thy neighbor economics.

Essentially, they are willing to burn all sorts of rules and regulations in order to beat a neighboring state to attract businesses and individuals. They then make up for this loss by drawing federal dollars that are usually paid for by people and companies in those same states they drew people and businesses away from.

But they are absolutely willing to regulate where it helps them individually even if it is against purported principles.

And then, of course, there’s the annual hypocrisy where red state senators will decry any funding towards blue states (for example, emergency funding to California for fire disasters) but jump right into demanding as much money as they can get when there are disasters in their states.

Texas’s energy supply bounties hides this there but it’s evident all over the remaining red states.

How is it legal for them to regulate interstate commerce?

Probably not. But especially in emergencies it's often better to ask for forgiveness than to ask for permission. (Also, domestic in-state political signaling.)

Anyhow, the legal effect may be irrelevant if the gas producers wouldn't bother trying to export. I would imagine that producers are having trouble just delivering on their long-term, fixed-price contracts in Texas. The short-term prices might be so jacked up precisely because there's little to no surplus to export, period.

Texas Railroad Commission has been around since before relevant federal statutes were passed